This invention relates to an apparatus and method for forming three-dimensional articles from thermoplastic sheet material and more particularly, to a thermoforming apparatus wherein a solid state sensor is provided along with electronic circuitry to control the clamping and cutting of thermoplastic material within the apparatus.
Thermoforming apparatus capable of forming thousands of plastic articles by pressure differential and match die forming techniques on a continuous basis are well-known. An example of such forming equipment is illustrated in U.S. Pat. No. 3,113,345, issued Dec. 10, 1963 to Butzko. In this device, a plastic sheet is advanced between a pair of relatively movable platens on a forming press. Heating means on one of the platens softens the web and positive fluid pressure, such as provided by forced air, is exerted through one of the platens to blow the web against the heated area of the other platen. Once the web has been softened, the air flow is reversed and the heated portion of the web is drawn by a vacuum into a mold cavity in the unheated platen and conforms closely to the walls thereof. Positive fluid pressure, such as provided by forced air, can be exerted through the opposite, heated platen, to aid in forming the web and conforming it to the mold cavity in the opposite platen. The relatively cold surface of the mold cavity cools the formed article.
Cutting means such as a peripheral knife is also provided on one of the platens. Initially, the knife clamps the web to the heated platen when the platens move downwardly relative to one another. Once the softened web is formed by drawing it into one of the mold cavities by the pressure differential technique referred to above, slight movement is given to the knife bearing platen to completely extend it and have it either cut completely through or score the periphery of the formed article in the thermoplastic sheet.
The prior art has used various mechanical methods for clamping the thermoplastic sheet and for severing the formed article from the sheet following the forming operation. Butzko accomplishes these two steps through the use of purely hydraulic and mechanical means. Air cylinders and plungers are used to provide means for limiting the closed relation of the platens to a point at which the thermoplastic sheet is clampedtherebetween without full penetration of the sheet by the peripheral knife. Additional hydraulic means are provided for rendering the limit means inoperative, permitting full closing movement of the platens and full penetration of the sheet by the knife. A timer controlled valve means is provided for establishing the proper timing of these steps during the dynamic operation of the apparatus.
Another method for providing the two stage closing motion of the platens is disclosed in U.S. Pat. No. Re. 26,413 to Keyes. This patent teaches the actuation of two air cylinders successively, to close the press in two stages. U.S. Pat. No. 3,166,790 also to Keyes shows the identical two cylinder press closing means.
The prior art thermoforming presses rely on strictly mechanical means for providing the two stage clamping and scoring/severing functions described above. Such mechanical systems require a relatively lengthy set-up time in order to properly adjust the point at which the blade will be stopped in clamped relation with the thermoplastic sheet. Different thicknesses of thermoplastic material will require varying extensions of the platens and blade in order to secure the proper clamping function.
In addition to the set-up time, it has been found that the prior art mechanically operated machines must be readjusted periodically throughout the day in order to continue proper operation.
These and related problems have been solved by the present invention which uses solid state sensing techniques and an electronically controlled clamping and severing apparatus in conjunction with an otherwise substantially conventional thermoforming apparatus.